1. Field of the Invention
The present invention relates to an optical scanning device which is configured to adjust a micro mirror incorporated in the device.
2. Description of the Related Art
The optical scanning device incorporated in an electrophotographic image forming apparatus can use a polygonal mirror that deflects a beam of light and causes the beam to repeatedly scan a photosensitive drum to form an electrostatic latent image on the photosensitive drum. The photosensitive drum is an image carrier that can rotate at a constant speed. A scanning operation in a longitudinal direction of the photosensitive drum is referred to as main scanning. The electrostatic latent image of a line formed in a main scanning operation is referred to as main scanning line.
There is a problem that the clearance (pitch) between main scanning lines is not uniform when the lines are formed on a photosensitive drum with a beam of light. This is referred to as pitch unevenness (or irregularity)
One of the reasons is that an actual rotational speed of the photosensitive drum is not constant. This is referred to as “rotation unevenness.” Another reason is that the tilt angle of each plane of a polygonal mirror is not uniform due to manufacturing errors. This is referred to as “plane tilt.”From these reasons, the scanning position of a light beam on a photosensitive drum tends to deviate.
As discussed in Japanese Patent Application Laid-Open No. 2004-37757, to solve the above-described problem, a conventional system uses a micro mirror that is disposed in optical path and can slant in a sub-scanning direction (i.e., a direction normal to the main-scanning direction). This system can correct the pitch unevenness of main scanning lines by adjusting the inclination of the micro mirror.
As discussed in Japanese Patent Application Laid-Open No. 2001-270153, a multi-beam image forming apparatus includes a galvanometer mirror that can correct a positional deviation between light beams. This apparatus can operate in a specific mode for adjusting an angle of inclination of the galvanometer mirror to eliminate a deviation if caused in both the main-scanning direction and the sub-scanning direction due to effects of drift.
The correction amount required for correcting the plane tilt of a polygonal mirror is different for each plane. Therefore, the number of angles of inclination of a micro mirror to be set for adjustment is equal to the number of planes of the polygonal mirror.
FIG. 8 illustrates an exemplary polygonal mirror 205 that has six reflection planes defined by a plane tilt profile described below. The first plane 205A has a tilt amount of −3 μm. The second plane 205B has a tilt amount of −1 μm. The third plane 205C has a tilt amount of 0 μm. The fourth plane 205D has a tilt amount of 2 μm. The fifth plane 205E has a tilt amount of −1 μm. The sixth plane 205F has a tilt amount of −2 μm.
In the expression of the plane tilt, the tilt amount indicates a deviation caused by the plane tilt of the polygonal mirror 205. For example, the expression “tilt amount of 2 μm” indicates that a light spot formed on a photosensitive drum deviates from a reference position (i.e., ideal position) in the sub-scanning direction by an amount of +2 μm.
FIG. 9A is a graph illustrating a time sequential deviation in the sub-scanning position relative to the reference position (i.e., ideal position), according to the polygonal mirror 205 having the plane tilt profile illustrated in FIG. 8. T represents the period of time required for a main scanning operation. One complete rotation (i.e., rotation of 360°) of the polygonal mirror 205 requires the time of 6 T (=6 planes×T) in total.
When the scanning operation uses the first plane 205A, the sub-scanning position (i.e., the position of the light spot formed on a photosensitive drum) deviates from the reference position by an amount of −3 μm, due to the plane tilt of the polygonal mirror 205. After time elapse of T, when the scanning operation uses the second plane 205B, the sub-scanning position deviates from the reference position by an amount of −1 μm. After time elapse of T, when the scanning operation uses the third plane 205C, the sub-scanning position coincides with the reference position.
Furthermore, after time elapse of T, when the scanning operation uses the fourth plane 205D, the sub-scanning position deviates from the reference position by an amount of 2 μm. After time elapse of T, when the scanning operation uses the fifth plane 205E, the sub-scanning position deviates from the reference position by an amount of −1 μm. After time elapse of T, when the scanning operation uses the sixth plane 205F, the sub-scanning position deviates from the reference position by an amount of −2 μm. In this manner, the sub-scanning position deviates time-sequentially during one complete rotation of the polygonal mirror 205 that requires the time of 6 T in total.
To correct the sub-scanning positional deviation (i.e., effects of plane tilt) illustrated in FIG. 9A, it is useful to slant (incline) the micro mirror at a predetermined angle of inclination at which the effects of the plane tilt of the polygonal mirror can be cancelled. Namely, as illustrated in FIG. 9B, the inclination of the micro mirror can be adjusted so that a beam of light, after it falls on the polygonal mirror, generates a deviation opposed to the sub-scanning positional deviation illustrated in FIG. 9A. FIG. 9C illustrates an ideal state where the deviation in the sub-scanning position is completely suppressed.
However, the above-described correction system is subjected to the following problems. The micro mirror is not free from effects of environmental changes (e.g., ambient temperature change as well as temperature change of the micro mirror itself). The angle of inclination of a micro mirror changes according to a temperature change as illustrated in FIG. 9B′. Namely, if any change occurs in the environmental conditions, the micro mirror cannot hold its angle of inclination at a desired value (i.e., an angle to be set in response to an applied input voltage) when the input voltage corresponding to the correction illustrated in FIG. 9B is applied to the micro mirror. FIG. 9C′ illustrates a state where the deviation in the sub-scanning position is not completely removed.
The above-described problem (i.e., the angle of inclination of a micro mirror deviates from an ideal angle corresponding to a specific input voltage due to environmental change) is described below in more detail. When the scanning operation uses the first plane 205A of the polygonal mirror 205, even if an input voltage corresponding to the tilt amount of +3 μm is applied to the micro mirror, an actual sub-scanning positional deviation becomes +2.3 μm due to the environmental change. Similarly, when the scanning operation uses the second plane 205B, an actual sub-scanning positional deviation becomes 0.8 μm if the input voltage corresponding to the tilt amount of 1 μm is applied to the micro mirror.
When the scanning operation uses the third plane 205C, an actual sub-scanning positional deviation becomes 0 μm (because the third plane 205C requires no correction). When the scanning operation uses the fourth plane 205D, an actual sub-scanning positional deviation becomes −1.5 μm. When the scanning operation uses the fifth plane 205E, an actual sub-scanning positional deviation becomes 0.8 μm. When the scanning operation uses the sixth plane 205F, an actual sub-scanning positional deviation becomes 1.5 μm.
Namely, according to the example of FIG. 9B′, if any change occurs in the environmental conditions, the angle of inclination of a micro mirror decreases compared to an angle of inclination to be obtained when an input voltage is applied before the environmental conditions change, even if the micro mirror slants (inclines) in response to the input voltage so as to eliminate a plane tilt amount of the polygonal mirror.
Therefore, as illustrated in FIG. 9C′, when the scanning operation uses the first plane 205A of the polygonal mirror 205, an error of −0.7 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the second plane 205B, an error of −0.2 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the third plane 205C, no error is generated in the sub-scanning positional deviation (because the third plane 205C requires no correction).
When the scanning operation uses the fourth plane 205D, an error of 0.5 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the fifth plane 205E, an error of −0.2 μm is generated in the sub-scanning positional deviation. When the scanning operation uses the sixth plane 205F, an error of −0.5 μm is generated in the sub-scanning positional deviation. In this manner, the conventional correction system cannot sufficiently correct the sub-scanning positional deviation.